In systems according to the mobile cellular standard known as 3G Long Term Evolution or 3G LTE for short, so called Multiple Input Multiple Output, MIMO, transmission techniques like transmit diversity, multi-stream and beam forming are expected to be an integral part of the systems. These MIMO techniques exploit multiple transmit antennas at the eNodeB, the evolved NodeB, sometimes also referred to as “the radio base station”. It is also expected that a large percentage of the user terminals, also called UEs, “user equipment”, will be equipped with multiple antennas in order to enable them to utilize MIMO transmission techniques, both in the downlink and the uplink directions.
A further definition of MIMO is as follows: a channel with multiple antennas on the transmit side as well as on the receive side is referred to as MIMO, Multiple Input Multiple Output. A MIMO channel can be defined as NTX×NRX, where NTX is the number of transmitter antennas and NRX is the number of receiver antennas.
The term “rank” is often used in connection with MIMO techniques, and can be defined as follows: a rank of 1 means that only one data stream is transmitted, and a rank of n means that n data streams are transmitted simultaneously. A rank of 1 corresponds either to transmission from a single antenna or more traditional beam forming from multiple antennas, in which case the phase of the signals transmitted from the transmit antennas is adapted so that the signals add up coherently at the receiver. Phase adaptation typically requires feedback from the receiver regarding the rank with which it is possible and/or desirable to transmit, and usually requires updates on a rather short time scale, e.g. in the order of a few milliseconds. This can also be seen as a general observation on MIMO systems: in a MIMO system, the rank, i.e. the number of streams that can be supported varies with the radio channel, and needs to be fed back from the receiver to the transmitter. This adaptation of the number of streams is also called rank adaptation, and, as pointed out above, needs to be done on the time scale mentioned above, i.e. in the order of a few milliseconds.